# CVE-2026-54743: Lemmy: Stored XSS via markdown image alt-text in lemmy-ui html5-embed

> Stored XSS in Lemmy allows attackers to execute JavaScript in viewers' browsers via crafted markdown image alt-text, potentially leading to session hijacking.

- **Severity:** Medium 6.4 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 19 Aug 2026
- **Updated:** 21 Aug 2026
- **Also known as:** GHSA-2g66-9fr3-ppwj

## What is CVE-2026-54743?

CVE-2026-54743 is a medium-severity cross-site scripting vulnerability in LemmyNet lemmy, published on 19 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

Lemmy is a link aggregator and forum for the fediverse. Prior to lemmy-ui 0.19.19-beta.1, LemmyNet/lemmy-ui renders Markdown in src/shared/markdown.ts for post bodies, comment bodies, private messages, and community and site sidebars through mdToHtml, which returns a raw __html object that Inferno injects without a sanitizer pass. setupMarkdown configures html as false but registers markdown-it-html5-embed@1.0.0 with useImageSyntax enabled, so an image targeting video media becomes a video element whose fallback incorporates the image alt text through unescaped string replacement. The html setting does not apply to plugin-generated output, allowing crafted alt text to reach the DOM as live HTML in contexts that do not use mdToHtmlNoImages. An approved member or a remote federated instance can store such content, and a viewer who renders it may execute JavaScript in the lemmy-ui origin, exposing the viewer's session and authenticated actions. The advisory notes that Content Security Policy prevents the described exploit in production, but also states that the tested default self-hosted deployment serves no Content-Security-Policy. This issue is fixed in lemmy-ui version 0.19.19-beta.1.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2026-54743?


## How do I fix CVE-2026-54743?

No fix has been published for CVE-2026-54743 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-54743 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-54743 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-54743 and fix it if it is affected.

What it is: CVE-2026-54743 is a medium-severity cross-site scripting vulnerability in LemmyNet lemmy, published on 19 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-54743 exists.

Affected product: LemmyNet lemmy. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-54743/ (Markdown: https://insights.sbomify.com/CVE-2026-54743/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-54743
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-54743
- OSV: https://osv.dev/vulnerability/CVE-2026-54743

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-54743/
Summaries are generated and may contain mistakes; the official description above is the record.